1. Field of the Invention
The present invention relates to process controllers. More specifically, the present invention is directed to a process control system having a backup controller to be used as a substitute for a failed primary controller.
2. Description of the Prior Art
The use of process controllers to control the functioning of an industrial process is well-known in the art and involves a monitoring an adjustment of many critical process variables. In order to improve reliability of the operation of such process controllers inasmuch as the failure of an on-line industrial process controller would produce a catastrophic effect in the monitored and controlled industrial process, it is desirable to provide a means for counteracting failure of a process controller as quickly as possible. While a manual substitution by an operator of known good controller for a failed controller would quickly affect a repair, such a substitution is often impractical to go into the remote location of many industrial process controllers which are generally located at the site of the industrial process being monitored and controlled rather than in a convenient centralized location. Further, in many continuous industrial processes it is desirable to maintain the operation of the industrial process rather than effecting a shut-down following a detection of a failed process controller. Finally, the contemporary use of digital computer apparatus and techniques in the process control field involves the use of digital communication systems to the process controller as well as digital output signals from the process controllers to the digital communication system. Thus, the digital controller must respond to a digital identification, or communication, signal as well as providing digital process control and a monitoring information to the digital communication system. Accordingly, a substituted process controller must be prepared to assume all of the functions of the failed controller as well as being responsive to the communication system digital address code for the failed controller.